1. Field of the Invention
The present invention relates to a thermal head driving integrated circuit (IC) for controlling activation of a plurality of heating resistors.
2. Description of the Related Art
FIG. 2 is a circuit diagram showing an example of a conventional thermal head driving IC.
In the thermal head driving IC, a heating resistor 2 is connected to a heating resistor power supply 1, and the heating resistor 2 is connected to an output terminal 3 for the thermal head driving IC. The driving NMOS transistor 5 is disposed between the output terminal 3 and a ground 4. When a gate terminal 6 of the driving NMOS transistor 5 is set high, the driving NMOS transistor 5 is turned on and current flows through the heating resistor 2 to generate heat. Also, when the gate terminal 6 of the driving NMOS transistor 5 is set low, the driving NMOS transistor 5 is turned off and no current flows through the heating resistor 2 to generate no heat.
If the on-resistance of the driving NMOS transistor 5 is high when the driving NMOS transistor 5 is turned on and a current flows through the heating resistor 2 to generate a heat, voltage drop occurs in the driving NMOS transistor 5 causing decrease in voltage to the heating resistor 2 and eventually insufficient heating. An increase in a channel width of the driving NMOS transistor 5 reduces the on-resistance of the driving NMOS transistor 5, however, such results in a chip size of the thermal head driving IC becoming larger, leading to a problem of increase in cost.
In order to increase a driving-capability of a MOS transistor without increasing the channel width thereof, in the conventional art, for example, there is disclosed a method of applying a voltage such that a substrate potential of the MOS transistor is biased against the source in a forward direction of a PN diode, thereby turning on a parasitic bipolar transistor (for example, see JP 2004-15402 A).
However, in the method of biasing the substrate potential of the MOS transistor in a forward direction against the source, it is inevitable to supply the base current for the parasitic bipolar transistor in order to bias the substrate. Consequently, a new problem arises in that a current consumption of the IC increases. The parasitic bipolar transistor has a large base width, and the hFE thereof becomes considerably larger compared to a standard bipolar transistor, and thus an increase in the current consumption becomes a major problem.